The present invention relates generally to electrical switches and more particularly to piezoelectric switches.
Piezoelectric switches, also commonly referred to simply as piezo switches, are well known and widely used in the electrical switch industry.
A piezoelectric switch traditionally includes a piezoelectric element comprising a disc-shaped piezoelectric plate and a disc-shaped conductive plate that are affixed together by an adhesive. The piezoelectric plate is commonly constructed out of piezo crystal and serves as the positive terminal for the switch. The conductive plate is typically constructed out of a conductive metal, such as brass, and serves as the negative terminal for the switch. Each of the above-described plates of the piezoelectric element is electrically connected to a printed circuit board which in turn supports the output prongs (i.e., conductive terminals) for the switch.
The piezoelectric element and printed circuit board of a piezoelectric switch are typically mounted within a hollowed out, non-conductive protective housing. Preferably, the housing is provided with an enlarged, button-shaped touch plate which serves as the contact surface for manually regulating the state of the switch. Typically, the piezoelectric plate of the piezoelectric element is affixed to the inner surface of the touch plate using double-sided adhesive tape which is often of a suitable thickness so as to serve as an insulating spacer. Accordingly, it is to be understood that the application of tactile pressure onto the outer surface of the touch plate is transferred to the piezoelectric element which, in turn, causes the piezoelectric element to very slightly deform. In response to its slight deformation, the piezoelectric element generates an electrical signal that is sent to the printed circuit board. If the electrical signal received by the printed circuit board meets a minimum threshold, the printed circuit board in turn switches the state of connection between the pair of output prongs for the switch (i.e., between an open state and a closed state).
An example of a piezoelectric switch is disclosed in U.S. Pat. No. 6,064,141 (hereinafter the '141 patent) which issued on May 16, 2000 in the name of Richard D. Wiciel, said patent being incorporated herein by reference. In the '141 patent there is disclosed a piezoelectric switch that comprises a housing having an inner surface and an outer surface. A piezoelectric element is mounted on the inner surface of the housing. The piezoelectric element has a top layer made of piezo crystal and a bottom layer made of a conductive material such as brass. A printed circuit board is disposed within the housing for controlling the state of the switch, the printed circuit board being electrically connected to the piezoelectric element by an electrical conductor. In one embodiment, the electrical connector is in the form of a flex circuit which has a positive terminal, a negative terminal and a printed circuit board overlay portion that is affixed to the printed circuit board. The flex circuit sandwiches the piezoelectric element therewithin so that the positive terminal contacts the top layer and the negative terminal contacts the bottom layer. In another embodiment, the piezoelectric switch additionally comprises a pushbutton assembly movably mounted on the housing for providing the operator of the switch with a noticeable indication of a change in the state of the switch.
Other known patents of interest include U.S. Pat. No. 4,430,595 which issued on Feb. 7, 1984 in the names of H. Nakasone et al., U.S. Pat. No. 4,761,582 which issued on Aug. 2, 1988 in the name of J. M. McKee, U.S. Pat. No. 5,332,944 which issued on Jul. 26, 1994 in the name of D. J. Cline, U.S. Pat. No. 5,231,326 which issued on Jul. 27, 1993 in the name of J. C. Echols, U.S. Pat. No. 5,442,150 which issued on Aug. 15, 1995 in the name of R. G. Ipcinski, and U.S. Pat. No. 5,636,729 which issued on Jun. 10, 1997 in the name of R. Wiciel, all of said patents being incorporated herein by reference.
Piezoelectric switches of the type as described above offer a number of notable advantages over traditional electrical switches which include, among other things, multiple moving mechanical parts (e.g., pivotable contact arms).
As a first advantage, piezoelectric switches of the type described above include no moving mechanical parts. Because it has been found that moving mechanical parts often falter from use over time, piezoelectric switches are consequently more reliable than traditional, mechanically-based, electrical switches, which is highly desirable.
As a second advantage, piezoelectric switches of the type as described above can be hermetically sealed to protect against harmful environmental conditions (e.g., thermal influences and/or moisture). Specifically, because piezoelectric switches include no moving mechanical parts, the interior cavity of the housing for a piezoelectric switch can be filled with a potting compound, such as a silicon, to protect the electrical components of the switch from corrosive or otherwise damaging elements, which is highly desirable.
Although well known and widely used in commerce, piezoelectric switches of the type as described above have been found to suffer from a notable drawback. Specifically, it has been found that the use of double-sided tape to affix the piezoelectric element to the inner surface of the touch plate can compromise the success rate of the switch in use. In particular, it has been found that operation of a piezoelectric switch is optimized when the application of an input force onto the contact plate in turn deforms the approximate center of the piezo crystal. However, because the entire surface of the piezo crystal is evenly affixed to the inner surface of the contact plate by an adhesive (which may, in turn, weaken over time), the application of an input force onto the housing is just as likely to cause the outer periphery of the piezo crystal to deform as it is to cause the approximate center of the piezo crystal to deform, which is highly undesirable.